Fossil fuel is a general term for buried combustible geologic deposits of organic materials, formed from decayed plants and animals that have been converted to crude oil, coal, natural gas, or heavy oils by exposure to heat and pressure in the earth's crust over hundreds of millions of years.
Fossil fuel, also known as mineral fuel, is used synonymously with other hydrocarbon-containing natural resources such as coal, oil and natural gas. The utilization of fossil fuels has enabled large-scale industrial development and largely supplanted water driven mills, as well as the combustion of wood or peat for heat. Fossil fuels are a finite, non-renewable resource.
When generating electricity, energy from the combustion of fossil fuels is often used to power a turbine. Older generations often used steam generated by the burning of the fuel to turn the turbine, but in newer power plants, the gases produced by burning of the fuel turn a gas turbine directly. With global modernization in the 20th and 21st centuries, the thirst for energy from fossil fuels, especially gasoline derived from oil, is one of the causes of major regional and global conflicts.
The burning of fossil fuels by humans is the largest source of emissions of carbon dioxide, which is one of the greenhouse gases that allows radiative forcing and contributes to global warming. In the United States, more than 90% of greenhouse gas emissions come from the combustion of fossil fuels. In addition, other air pollutants, such as nitrogen oxides, sulfur dioxide, volatile organic compounds (VOCs) and heavy metals are produced.
Human activity raises levels of greenhouse gases primarily by releasing carbon dioxide from fossil fuel combustion, but other gases, e.g., methane, are not negligible. The concentrations of several greenhouse gases have increased over time due to human activities, such as burning of fossil fuels and deforestation leading to higher carbon dioxide concentrations. According to the global warming hypothesis, greenhouse gases from industry and agriculture have played a major role in the recently observed global warming.
Increased demand for energy by the global economy has also placed increasing pressure on the cost of hydrocarbons. Aside from energy, many industries, including plastics and chemical manufacturers, rely heavily on the availability of hydrocarbons as a feedstock for their manufacturing processes. Cost-effective alternatives to current sources of supply could help mitigate the upward pressure on energy and these raw material costs.
Algal fermentation for the production of biodiesel (fatty acid alkyl esters) is described in Li et al. (2007) Biotechnology and Bioengineering 98: 764-771; and in Xiong et al. (2008) Biotechnological Products and Process Engineering 78:29-36. These papers describe the growth of the green alga Chlorella protothecoides under heterotrophic conditions, the isolation of lipids from the harvested cells, and the production of biodiesel from the extracted lipids by transesterification using an immobilized lipase from Candida sp. 99-125. PCT Publication No. WO2008/151149 discloses microbial strains for the production of various biofuel components, including algal strains genetically engineered for the production of lipid. In some disclosed embodiments, the engineered microorganism includes a lipase gene, in which the expressed lipase acts as a transacylase to convert lipids (triacylglycerols) to biodiesel (fatty acid alkyl esters). The disclosure provides that the expression of the lipase gene can be delayed until after the growth period or can be expressed in an intracellular compartment, where it remains separate from the majority of the microalgal lipid (the lipase substrate) until transesterification, preferably after water has been substantially removed from the cells and/or an excess of alcohol has been added to the cells.
PCT Publication Nos. WO2007/136762 and WO2008/119082 disclose compositions and methods for producing biofuels that include recombinant microorganisms engineered for the fermentative production of fatty acid derivatives, such as, inter alia, fatty alcohols and wax esters. The host strain can include a plasmid that includes one or more fatty acid biosynthesis genes and can further include a lipase gene. PCT Publication No. WO2009/133351 discloses the use of plant cells grown in suspension tissue culture for the production of oil that can be used to make biodiesel. In some embodiments a lipase is added to the culture to reduce the glyceration level of the fatty acids produced by the cells.